In the field of high-voltage switches, it is desirable that a diode has a low reverse leakage current, high reverse voltage and a low forward turn-on voltage drop. Since power electronic devices based on wide bandgap semiconductor materials, particularly gallium nitride materials, have superior characteristics, gallium nitride Schottky diodes have been a hot topic in recent years. Currently, homoepitaxy of gallium nitride on gallium nitride substrates is still in a small-scale, high-cost stage. Although high-quality epitaxial materials and desired device performances can be achieved, such a technology has not been widely used due to high costs.
At present gallium nitride materials are mainly grown on heterogeneous materials, and have relatively high defect densities, e.g., 108 cm−3, thus desired performances still cannot be obtained for gallium nitride Schottky diodes having vertical structures. However, High Electron Mobility Devices (HEMTs) based on two-dimensional electron gas channels which have high electron mobility in the horizontal direction and are formed by aluminum gallium nitride/gallium nitride heterojunction structures have been widely used in the fields of RF and power electronics. This is because, on the one hand, gallium nitride is a kind of wide bandgap semiconductor materials which have critical breakdown electric field intensity about 10 times higher than that of silicon materials and thus has a characteristic of high reverse voltage, on the other hand, the two-dimensional electron gas channels can provide very low turn-on resistances so that power loss of the switching devices can be reduced. Therefore, horizontal diodes based on aluminum gallium nitride/gallium nitride heterojunction structures gradually become an important research topic in the industry.
For a Schottky diode formed by direct deposition of Schottky metal on an aluminum gallium nitride/gallium nitride heterojunction structure, a thickness of an aluminum gallium nitride barrier layer between the Schottky metal and the two-dimensional electron gas usually reaches to 20 nm, resulting in a large Schottky barrier thickness. In addition, a relatively large surface state density of the aluminum gallium nitride barrier layer will lead to the Fermi level pinning effect, which also results in a large Schottky barrier thickness. Therefore, the Schottky diode has a high forward knee voltage, e.g. greater than 1 V, which is disadvantageous for reduction of turn-on loss of the diode.
In order to reduce a forward turn-on voltage of the Schottky diode, anode groove structures are proposed. In such structures, an aluminum gallium nitride barrier layer and a portion of a gallium nitride channel layer in an anode region are etched and then deposited with an anode metal, so that the anode metal at sidewalls and the two-dimensional electron gas channel form metal-semiconductor contacts, which eliminates a Schottky barrier thickness formed by the aluminum gallium nitride barrier layer with a thickness of 20 nm and reduces the forward knee voltage, e.g. less than 0.7 V, of the diode. In addition, the two-dimensional electron gas channel having high electron mobility provides a very low turn-on resistance, so that a Schottky diode with high performances such as a low forward turn-on voltage and a low turn-on resistance can be obtained. Furthermore, the two-dimensional electron gas channel has a very low hole concentration due to the wide bandgap characteristic of the gallium nitride material, thus the diode has a very short reverse recovery time. However, the conventional gallium nitride Schottky diodes still have some shortcomings. For example, field-induced thermionic emission or electron tunneling effect in high electric fields will cause an increased reverse leakage current, which reduces the reverse voltage performance of the device.
In order to improve the performances of Schottky diodes, different structures have been proposed in some articles and patents.
For example, in the article “Fast Switching GaN Based Lateral power Schottky Barrier Diode with Low Onset Voltage and Strong Reverse Blocking” (IEEE ELECTRON DEVICE LETTERS, VOL.33, NO.3, MARCH 2012) by E. Bahat-Treidel et al., referring to FIG. 1, an anode 11 of a Schottky diode is designed as a structure of a groove plus a field plate. The medium under the field plate is a silicon nitride layer 12, metal in the groove of the anode 11 is directly in contact with two-dimensional electron gas 13. In this structure, the reverse voltage performance of the Schottky diode can be improved due to the field plate and an increased distance between the anode and a cathode.
Referring to FIG. 2, a Schottky diode comprising an anode 21 having two layers of composite dielectric layers is proposed in U.S. Pat. No. 8,772,842 B2 entitled “Semiconductor Diodes With Low Reverse Bias Currents” by Yuvaraj Dora et al. In this structure, the two layers of composite dielectric layers include a layer 21 designed as a field plate with a stepped shape and the other layer 23 as a passivation layer, which reduces a peak electric field intensity and increases a breakdown voltage.
Referring to FIG. 3, a Schottky diode with a structure having multiple steps is proposed in U.S. Pat. No. 7,898,004 B2 entitled “Semiconductor Heterostructure Diodes” by Yifeng Wu et al. In this structure, a single dielectric layer is used to form an anode 31 having a stepped field plate structure to reduce a peak electric field intensity. Metal in a bottom portion of a groove of the anode 31 forms a Schottky contact with semiconductor material 32, so as to form an anode structure.
In the above-mentioned technical solutions, field plates are added. Under an applied reverse bias voltage, a field plate can reduce a reverse leakage current of a Schottky diode by reducing an electric field intensity at a Schottky junction, and improves a breakdown voltage under a turn-off state of the Schottky diode. In practice, however, due to the presence of a passivation dielectric layer under the field plate, a reverse bias voltage applied to an anode will be fully applied to the reverse bias Schottky junction before depleting two-dimensional electron gas in a channel under the anode. In order to achieve an ideal passivation effect and a more optimized electric field mitigation effect by the field plate, the passivation dielectric layer usually has a thickness of about 100 nm, which is relatively large compared to an aluminum gallium nitride barrier layer which usually has a thickness of about 20 nm. In addition, currently silicon nitride which has a relatively small dielectric constant compared to aluminum gallium nitrogen is usually used to form the passivation dielectric layer, a relatively high voltage is required to deplete the two-dimensional electron gas. That is, before the two-dimensional electron gas is depleted and the field plate plays a role in mitigating the electric field, the Schottky junction has undergone a high reverse bias voltage. In this case, the field-induced thermionic emission and the tunneling effect both result in an increased reverse leakage current, thus the reverse leakage current is still at a relatively high level.
Therefore, it is required to further reduce the leakage current under a reserve bias state of the gallium nitride Schottky diode and improve the reverse voltage performance thereof.